NORTHCUTT, R.G.; University of California, San Diego: External taste buds in channel catfish are not neurally induced

The predominant theory of taste bud induction has been the neural induction theory, according to which the peripheral processes of the facial, glossopharyngeal and vagal nerves invade the oropharyngeal epithelium and induce taste bud primordia. There is growing evidence, however, that nerve fibers are not needed for the induction of taste buds. In a new theory of taste bud induction, termed the early specification theory, it is proposed that oropharyngeal epithelium is specified to form taste buds during gastrulation, and that taste bud primordia arise by cell-cell interactions within the specified epithelium. The gustatory system of channel catfishes provides a particularly robust model in which to test these theories. Catfishes have large numbers of external taste buds distributed over the head and trunk, and all taste buds on the trunk are innervated by a single pair of rami, temed the recurrent facial rami. If the recurrent facial rami are prevented from migrating onto the trunk in developing catfish embryos, normal taste buds still form in the absence of facial innervation. Although this experiment demonstrates that taste bud induction in these fishes is not due to facial innervation, it does not rule out the possibility that taste buds could be induced nonspecifically by other nerves such as the lateral line or spinal nerves. This possibility is ruled out, however, by culturing isolated pieces of non-innervated caudal fin, as taste buds develop normally in these cultures. These data clearly demonstrate that external taste buds are not neurally induced in channel catfishes.